Measurement of web thickness is important in many industrial process applications for various purposes such as, for example, process control or quality assurance. Various systems have been devised for measuring the thickness of running webs of material, and often such gauges are mounted such that they can traverse the moving web across its width to provide a profile of web thickness.
One of the techniques used heretofore in measuring web thickness has employed nuclear gauges. Such nuclear gauges usually include a beta source mounted on one side of the web and a detector on the other for detecting emitted radiation not attenuated by the web. Nuclear thickness gauges are preferred in some applications but they have their drawbacks. Among the disadvantages is the reluctance of certain industries to allow use of a source of nuclear radiation. A second feature which is objectionable in some industries is the fact that nuclear detectors primarily measure density, and therefore the thickness estimates they produce are indirect. Accordingly, if the density of the material being measured varies, the accuracy of the thickness measurements will also vary. Pass line variations, i.e., variation in the position of the moving web with respect to the detector, can also be a problem.
Conceptually, ultrasonic gauges can be useful in measuring web thickness since they are capable of accurately detecting the distance between the sensor and the surface of the web at which they are directed. However, when ultrasonic sensors are put into a traversing mechanism for scanning across the web, the expected variation (even in a high quality traversing arrangement) of the scanning head with respect to the web can introduce a degree of error which completely masks the nominal accuracy of the transducers. For example, if it were desired to scan a web of nominal 100 mil thickness with a 1% accuracy, it would be necessary to provide a traversing mechanism with a worst case positional variation of .+-.0.5 mils. However, the best case that might be expected is a variation in the range of .+-.10 mils or more, making it impossible to measure a 100 mil web with anything better than .+-.10% accuracy. Traversing ultrasonic gauges might be useful when measuring substantially thicker webs (on the range of inches) where a 10 mil variation in the traverse mechanism is a small percentage of the total thickness. In some cases, it is possible to produce an "air profile" intended to compensate for positional variations in traverse, but that approach presumes that the variations are repeatable from traverse to traverse, and does not take account of any non-systematic variations. Thus, at least for thin webs, ultrasonic sensors have not provided the simplicity combined with accuracy which has been desired.
Another type of thickness sensor which has been employed commercially is illustrated in Horn et al. U.S. Pat. No. 3,617,872, assigned to the same assignee as the present invention. The system disclosed in that patent utilizes an eddy current sensor mounted on one side of the web, a metallic support on the other side of the web, and an air cushion supporting the head containing the eddy current sensor at a predetermined distance above the web. The eddy current sensor detects the distance between itself and the metallic support and, based on the assumption that the air cushion is always of the same thickness, is therefore a measure of web thickness. Such a system is not as easily implemented as might be desired in many circumstances and is also of less accuracy than desired.
U.S. Pat. No. 4,311,392 relates to thickness measuring apparatus and proposes a system utilizing both laser optics and an eddy current detector in the same measuring head. The proposed system utilizes laser optics for detecting the position of the front surface of the web, a backing roll for supporting the rear surface of the web, and an eddy current detector for producing a measurement relating to the distance between the head and the surface of the backing roll. Outputs of the two detectors are combined to produce a measure of sheet thickness corrected for variations in positional relationship between the backing roll and the measuring head. Apparent problems with the system can arise from using relatively complex optical systems in an industrial environment and the necessity for keeping the optics clean. The requirement that the optical source and detector be separately positioned to employ substantially different optical paths not only makes the system complex and potentially difficult to align, but the manner in which the complex optical path is associated with the eddy current sensor can render the system subject to inaccuracies due to relatively minor mechanical misalignment.
U.S. Pat. No. 4,276,480 relates to various forms of thickness measuring apparatus. Among the diagrams is FIG. 7 which illustrates a two head system for sensing both sides of an unsupported web, but using relatively complex optical paths and indirect sensing of independent reference lines, requiring the use of four optical light paths in all. The complexity would appear to relate not only to setup and maintenance, but would likely also affect the accuracy obtainable from a device constructed as disclosed.
Many of the thickness measuring systems known heretofore have been specially suited to their particular environment but have had little flexibility in adaptation to meet differing requirements. Thus, for example, the system of the aforementioned U.S. Pat. No. 4,311,392 would not appear to be suitable to a simple, accurate non-backing roll environment. The system described in the aforementioned U.S. Pat. No. 3,617,872 is of limited flexibility because of requirement for both pneumatic and electronic systems and the backing plate for riding against the rear of the web.